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Three blistering fires are blazing through Wyoming's scenic Powder River Basin, but firefighters aren't paying any attention. Other than a faint hint of acrid odors and a single ribbon of smoke rising from a tiny crack beyond the nearby Tongue River, a long look across the region's serene grassland shows no sign of trouble.

That's what makes the three infernos, and the toxins they spew, so sinister. Their flames are concealed deep underground, in coal seams and oxygen-rich fissures, which makes containment near impossible. Shielded from fire hoses and aerial assaults, the flames are chewing through coal seams 20 feet thick, spanning 22 acres. They're also belching greenhouse gases and contaminants, contributing to an out-of-sight, out-of-mind environmental hazard that extends far beyond Wyoming's borders. "Every coal basin in the world has fires sending up organic compounds that are not good for you," says Mark Engle, a geologist with the U.S. Geological Survey who studies the Powder River Basin, "but unless you live close to them you probably never see them." See TIME's photoessay "Wildfires Burn Across California."

A surprising number of us live close to them. According to a review by the Department of Interior's Office of Surface Mining Enforcement and Reclamation, more than 100 fires are burning beneath nine states, most of them in Colorado, Kentucky, Pennsylvania, Utah and West Virginia. But geologists say many fires go unreported, driving the actual number of them closer to 200 across 21 states. Most have burned for years, if not decades. Pennsylvania's three dozen underground fires include America's most notorious subterranean blaze, a 48-year-old fire in Centralia, whose noxious emissions sickened residents and eventually prompted the federal government in the late 1980s and early '90s to evict homeowners and pay them a collective $40 million for what is now a virtual ghost town.

Internationally, thousands of underground coal fires are burning on every continent except Antarctica. Anupma Prakash, a University of Alaska at Fairbanks geologist who maps the fires, calls them "a worldwide catastrophe with no geographic territory, and if we don't take care of them they're going to take a toll on us." The problem is most acute in industrializing, coal-rich nations such as China, where underground fires are consuming at least 10 million tons of coal annually  and some estimates multiply that amount twentyfold. In India, 68 fires are burning beneath a 58-square-mile region of the Jhairia coalfield near Dhanbad, showering residents in airborne toxins. "Go there and within 24 hours you're spitting out mucous with coal particles," Prakash says. "It's bad, worse than any city, anywhere."

Quantifying the amount of pollution that underground fires create is as difficult as spotting them. The smoke escaping through vents contains carbon dioxide, methane, mercury and at least 40 toxic compounds  "a whole soup of nasty stuff," according to Glenn Stracher, a geologist at East Georgia College in Swainsboro, Ga., who studies the emissions. But that soup's ingredients vary from hour to hour, even vent to vent, and some of the gases also escape through adjacent soil. "It's not like the oil well in the Gulf, where you measure how much is coming out per unit of time," Stracher says. "Making calculations is a tricky business." He and other researchers readily admit that global coal fire emission estimates  40 tons of mercury spewing into the atmosphere annually, and 3% of the world's annual CO2 emissions  are imprecise. But the negative implications for human health and global warming, they say, are clear. See TIME's photoessay "American Inferno."

With that in mind, some countries are investigating whether snuffing the fires could help them earn carbon credits under the Kyoto Protocol, and are stepping up prevention awareness. Though geologic records show evidence of underground coal fires dating to the Pleistocene era, modern-day coal fires are often an unintended side effect of mining operations that open coal seams to oxygen. Once exposed, the coal undergoes a chemical reaction that releases heat. In some climate conditions the coal spontaneously combusts. Otherwise, lightning, wildfires or an ill-placed spark can trigger the blaze. The flames rip inside the buried coal seams at temperatures exceeding 1,000°F, sometimes fed by oxygen trapped in the microscopic spaces between dirt particles. They're also assisted by a process known as subsidence: as the burning coal turns to ash, it causes the overlying ground to crack and collapse, supplying the fire with fresh air. (See TIME's photoessay "Fire in the Heartland.")

Extinguishing a fire amounts to a frustrating, expensive version of whack-a-mole. "You put one down, then 300 feet later another one picks up," says Engle. Firefighters can dig up the burning coal or form a break around it, and they sometimes pump ignited seams with cryogenic liquids that absorb enough heat to prevent burning. But large coal deposits can span several miles at 90 feet deep and 100 feet thick, creating a honeycomb of pores that always leaves a few pockets of fire-fanning oxygen. That means the same fire can later reignite  and fighting underground fires doesn't come cheap. The U.S has already spent more than $1 billion battling underground coal fires, according to the Office of Surface Mining. Officials gave up hopes of dousing the extensive Centralia, Pa., fire because the job would have cost more than $600 million  and that was in 1983.

Less expensive alternatives are beginning to hit the market, from special heat-resistant grouts to a fire-smothering nitrogen foam, and other innovative solutions are on the way. "Look, tornadoes and earthquakes are here and gone," Stracher explains, "but these fires will burn for hundreds of years if we don't do something about them." It seems that where there's smoke there is indeed fire, whether we can see the hidden flames or not.